Hypostasis is a word used to refer to who God is. Since Tertullian, typically the Orthodox speak of the who of God as being three "persons" of Father, Son, and Holy Spirit. Note that While Person is not totally accurate, it is better than referring to each as "it"; so, for lack of better terminology, Person is the preferred terminology.
This use of "person" in English is influenced by the Latin persona wherein there could be some confusion and one must understand the underlying meaning. In Greek, "πρόσοπον" (prosopon) is the word for person. It comes from the name for the mask used in Greek theater and means "face". This mask represented the outward manifestation of the personality of the character. Υπόστασις (hypostasis) is the preferred Orthodox term, however due to the pervasive use of the Latin in English translations one will find the word “person” more frequently in English even in Orthodox parish texts. This is probably because there isn’t really an appropriate clear theological English word translation for hypostasis, so in most English theological texts hypostasis is most used as a transliteration of the Greek word. The confusion may arise because Sabellianism (also known as “Monarchial Modalism”) was the ancient heresy that God was One, but that He manifested Himself in three modes, or πρόσοπα (prosopa is the plural of prosopon). Therefore Father, Son, and Holy Spirit are not hypostases, but rather different faces of the same God manifested at diverse times and places. So, modalism teaches that God is One essence with three modes, faces, or characters that He plays, which is not the understanding of the Nicene Fathers. The Latin word persona is how prosopon is generally translated from the Greek. They translate hypostasis as persona as well which exacerbates the confusion. If one misunderstands the Latin persona as being equivalent to prosopon, the mask of a character which an actor is pretending to be, then one might make the mistake of Sabellius who used prosopa as meaning different faces of the One God and perhaps conclude a modalistic interpretation. However, within Roman Catholic teaching, the word persona when used in speaking about the Holy Trinity is understood to mean the same as the Greek hypostasis meaning that each Person of the Trinity is not just a manifestation, mode, face, or role of the one God, but Father, Son, and Holy Spirit are three Persons, meaning three Hypostases, with one Ousia or Essence (the Latins prefer Substantia or "Substance" vs. Essence). Therefore, it’s implied that the use of “person” in English could possibly cause confusion (not limited to modalism, but also that when we speak of persons in our colloquial usage it means separate individuals, which to the non-theologically trained could imply three Gods), but in the understanding of the Latin Church they use persona the same as how the Orthodox use the Greek word hypostasis. So, one must be aware of the underlying Trinitarian theology to avoid misinterpretation of the dogma due to the limitations of human language, therefore one must be immersed in the spiritual mentality or phronema (mindset) of the Church to ensure a proper understanding.Hypostasis is not to be confused with God's essence/nature (ousia, i.e. what God is), or God's attributes (i.e. what God does).
- J.P. Farrell, "A Theological Introduction to the Mystagogy of St. Photios," in The Mystagogy of the Holy Spirit, ed. First M. Last Name (Brookline, MA: Holy Cross Orthodox Press, 1987), pp. 17-56.